Add Regexp-Regexp Concat operator for Grammar DSL

Author: vkutuev

solver/src/main/kotlin/org/ucfs/grammar/combinator/extension/StringExtension.kt

@@ -12,6 +12,7 @@ object StringExtension {
             .fold(initial) { acc: Regexp, i: Term<String> -> Alternative.makeAlternative(acc, i) }
     }
     infix operator fun Regexp.times(other: String): Concat = Concat(head = this, Term(other))
+    infix operator fun Regexp.times(other: Regexp): Concat = Concat(head = this, other)
     infix operator fun String.times(other: String): Concat = Concat(head = Term(this), Term(other))
     infix operator fun String.times(other: Regexp): Concat = Concat(head = Term(this), other)